Embodiments of the present invention relate to a DC-DC converter. Further embodiments relate to a method for operating a DC-DC converter. Some embodiments relate to a concept for resonant (switching) loss reduction of a bidirectional cascaded buck-boost converter.
Nowadays, for obtaining stable and highly efficient energy supply, many regenerative energy systems are designed by a bidirectional DC-DC converter with intermediate energy storage elements. The same have the function of adjusting different voltage levels. Accordingly, it can happen in many applications that the ranges of input and output voltage of the DC-DC converter overlap when charging and discharging the energy storages. Consequently, a specific type of DC-DC converters is necessitated, which can be operated both in the boost mode and in the buck mode.
Due to its simple topology and high performance, the cascaded buck-boost converter switch four switches is frequently used for such applications. As an interface between regenerative energy systems and energy storages, the efficiency of the DC-DC converter has a significant influence on the efficiency of the overall system. Consequently, concepts for minimizing power losses in the cascaded buck-boost converter are urgently needed.
U.S. Pat. No. 6,166,527 presents a control concept for the buck-boost converter for reducing power losses. Depending on the ratio of input and output voltage, the DC-DC converter is operated in three different modes, buck mode, boost mode, or buck-boost mode. Thereby, power losses of the DC-DC converter are reduced during buck and boost modes, but due to the simultaneous operation of the four switches, losses in the buck-boost mode still remain too high.
US 2012/0146594 A1 presents a control concept for the cascaded buck-boost converter with four switches. With the help of the concept, the cascaded buck-boost converter can operate in five different modes. These are the buck mode, the buck mode at half the switching frequency, the buck-boost mode at half the switching frequency, the boost mode at half the switching frequency and the boost mode. With approximating input and output voltages, the switching frequency of the PWM signals by which the switches are controlled is halved. Thereby, the range of buck and boost modes can be expanded and the range of the lossy buck-boost modes can be shortened accordingly. On the other hand, theoretically, the switching losses can be reduced by 50% in the range where the switching frequency is reduced by half. In summary, it can be stated that by using this concept, the efficiency of the DC-DC converter can be improved in the transition phase between buck mode and boost mode. However, in this concept, the switches are still hard switched such that the efficiency of the buck-boost converter is greatly limited.